1. Field of the Invention
The present invention relates to a rubber molded product containing an inorganic filler.
Since the rubber molded product of the present invention is excellent in thermal conductivity and flexibility, it is suitable as a radiating electronic member used in contact with heat-generating electronic parts in order to remove heat generated from an electronic instrument having the heat-generating electronic parts such as a transistor, a thyristor and the like.
2. Description of the Prior Art
A heat-generating electronic part such as a transistor or a thyristor generates heat during its use, and it is important to remove such heat. Heretofore, as a method for removing the generated heat, it has been common to remove the heat by attaching the heat-generating electronic part to a radiating fin or a metal plate with an electrically insulating heat conductive (radiating) sheet interposed therebetween. As such a heat conductive sheet, a relatively hard radiation sheet having a heat conductive filler such as boron nitride powder dispersed in silicone rubber has been mainly employed.
Recently, circuits have been highly integrated year by year, and accordingly their heat-generating amounts have been increased. Thus, a radiating member having a thermal conductivity higher than heretofore, is demanded, but it is required to avoid application of strong pressure in order to prevent a heat-generating electronic part from being damaged. Such being the case, a sheet-like radiating member having a very small thermal resistance and a high flexibility is demanded.
In order to make a radiating sheet highly thermally conductive, it is proposed to load a large amount of a highly thermally conductive filler, but in such a method, there is a disadvantage that a flexibility of a radiating sheet is remarkably lost. Therefore, it is proposed to load boron nitride particles in the direction of sheet thickness in such a manner as "standing state" by utilizing an anisotropic property of thermal conductivity of boron nitride particles (i.e. specific property that thermal conductivity of scale-like particles of boron nitride is very large in the direction of length) (see, for example, JP-A-62-154410, JP-A-3-151658 and JP-A-8-244094).
However, in the production method of JP-A-62-154410, it is necessary to use a special device such as an ultrasonic shaker, and it is difficult to apply to a thick sheet. It is disclosed in this publication that a thermal resistance of a sheet containing 46 to 56 vol % of boron nitride is 0.40.degree. C./W (sheet thickness: 0.45 mm) at minimum.
The production method of JP-A-3-151658 is a batchwise method, and accordingly a production cost is high. It is disclosed in this publication that a thermal resistance of a sheet containing 39 vol % of boron nitride or 56 vol % of boron nitride is 0.41.degree. C./W (sheet thickness: 0.5 mm ) or 0.30.degree. C./W (sheet thickness: 0.5 mm).
The production method of JP-A-8-244094 is an extrusion-molding method which can be continuously conducted, but a sheet thickness is controlled by the size of a mold outlet, and it is therefore hardly applicable to a sheet different in thickness. Moreover, a sheet produced by this method has boron nitride particles in the vicinities of upper and lower layers oriented in "laid state" along the sheet surface in respect to the thickness direction, and consequently a thermal conductivity is not satisfactorily raised. That is, it is disclosed in this publication that a thermal resistance of a sheet having a high boron nitride content of 50 vol % or 60 vol % is 0.20.degree. C./W (sheet thickness: 0.3 mm) or 0.12.degree. C./W (sheet thickness: 0.3 mm).
Since all of the above mentioned methods employ a large amount of expensive boron nitride powder, a cost of sheet produced therefrom becomes high, and a flexibility of a sheet becomes poor although it has a high thermal conductivity (low thermal resistance).